1. Field of the Invention
The present invention relates to a cooling device used for an image forming apparatus such as a printer, a facsimile, and a copying machine and to an image forming apparatus including the cooling device.
2. Description of the Related Art
As an image forming apparatus, well known is the one in which an electrophotography technology is used to form a toner image on a sheet of paper serving as a sheet-like member; and the toner is melted and fused by passing through a fixing device. As a printing speed becomes faster, has known is an image forming apparatus including a cooling device for quickly cooling down a sheet of paper heated by a fixing device.
Japanese Patent Application Laid-open No. 2006-003819 discloses an image forming apparatus in which a cooling device is provided at a downstream side of a fixing device in a sheet carrying direction. The cooling device includes a cooling roller for coming into contact with a sheet of paper, carrying and cooling down the sheet of paper. In the cooling device, two rollers are provided at an interval in the sheet carrying direction and extend thereon a carriage belt made of an elastic member for carrying the sheet of paper. One roller extending the carriage belt thereon rotates the carriage belt as a driving roller. Moreover, a cooling roller is provided between the two rollers extending the carriage belt thereon so as to come into contact with a top surface of the carriage belt. The cooling roller is turned around to rotate by power that conveys the carriage belt. The sheet of paper, heated to a high temperature while passing through the fixing device, is held and carried on the carriage belt to a nip region formed by the cooling roller and the carriage belt coming into contact with each other, where the sheet of paper is brought into contact with the cooling roller, so that heat of the sheet of paper is absorbed by the cooling roller and the sheet of paper is cooled down.
In order to suppress meandering of the carriage belt, there is a driving roller for rotating the carriage belt that has a so-called crown-like shape. That is a drum-like shape in which a diameter at an axial center part of the driving roller is larger than that at both ends thereof. With the driving roller having a crown-like shape, it becomes difficult for the carriage belt to move beyond the axial center part of the driving roller in an axial direction of the driving roller, so that meandering of the carriage belt can be suppressed.
However, since the crown-like shape driving roller has different diameters at the center part and at the end parts of the driving roller, tension of the carriage belt at the axial center part of the driving roller becomes larger than tension of the carriage belt at the axial end parts of the driving roller, which causes a difference in tension of the carriage belt in the axial direction of the driving roller. The difference in tension of the carriage belt causes a difference in pressure against the sheet of paper from the carriage belt in the axial direction of the driving roller when the sheet of paper is held by the cooling roller and the belt. The difference in pressure causes a problem of unevenness in gloss of an image on the sheet of paper.